As expected, Adobe patched a zero-day vulnerability in its popular Adobe Reader software Tuesday, marking the second time in three months that it delivered an update on the same day Microsoft issued its monthly fixes.

But while Microsoft's PowerPoint patch received lots of attention, the Adobe update should be at the top of people's to-do list, a security expert said today.

"Adobe's is more important than Microsoft's," said Wolfgang Kandek, chief technology officer at Qualys. "Even though Microsoft's had more visibility, if you have to choose between the two, you should patch Adobe. [Reader] is pretty much everywhere, attackers are increasing exploiting it and [PDF] is a widely-used corporate format."

The Reader vulnerability has been patched for all platforms in the free Reader 8 and 9, as well as in the corresponding versions of Adobe Acrobat, the for-pay PDF creation and editing application, the company said Tuesday.

Adobe's security advisory did not spell out the nature of the vulnerability, but earlier the company's security team had recommended that users disable JavaScript, indicating that the flaw was another bug in those applications' implementation of the scripting language.

Lack of detail is nothing new for Adobe's advisories; in March, when it patched another JavaScript bug in the same software, it also refused to provide anything more than general information because, while it had issued some updates, it had not managed to patch every version.

This week, Adobe updated Reader and Acrobat 9 to version 9.1.1, Reader and Acrobat 8 to version 8.1.5, and Reader and Acrobat 7 to version 7.1.2. But just as Microsoft did with its PowerPoint patch, Adobe stiffed Mac users. "Adobe expects to make available Adobe Reader 7 and Acrobat 7 updates for Macintosh before the end of June," the company said.

Adobe has staged updates in the past; in March the company said that waiting until all patches were ready for the February bug "just didn't make sense."

According to data from Finnish security company F-Secure, patching PDF bugs is critically important. So far this year, of the 663 targeted attacks F-Secure's uncovered -- targeted meaning attacks aimed at specific organizations or companies, or even individuals -- nearly half of them have relied on exploits of Adobe Reader vulnerabilities.

In the first four months of 2009, 48.9% of all targeted attacks involved a malicious PDF file attached to a legitimate-looking e-mail, said F-Secure, a dramatic change from 2008, when PDFs made up just 28.6% of target attacks. Previously, exploits of bugs in Microsoft Office's applications -- Word, Excel and PowerPoint -- made up the bulk of attacks.

"Why has it changed? Primarily because there have been more vulnerabilities in Adobe Acrobat/Reader than in the Microsoft Office applications," said Patrik Runald, chief research advisor at F-Secure, in a post to the company's blog last week.